Practice: Which one of the following combinations does not create a buffer?
A buffer is a solution composed of a weak acid with its conjugate base.
Concept #1: Understanding a Buffer.
Buffers resist drastic changes to the pH if a strong acid or strong base is added.
If a strong base is added then the buffer resists a pH change by having the weak acid neutralize it.
If a strong acid is added then the buffer resists a pH change by having the conjugate base neutralize it.
Practice: Which one of the following combinations does not create a buffer?
Practice: Which of the following combinations can result in the formation of a buffer?
As we stated earlier a buffer is composed of a weak acid and its conjugate base, but there are actually three ways to create a buffer.
Concept #2: Method 1 for Creating a Buffer.
The first and most obvious way to create a buffer is to simply combine a weak acid and its conjugate base. In this case, a buffer is most ideal when both components are highly concentrated and equal to one another.
The weak acid and conjugate base can be different from one another by up to a magnitude of 10. This is called the buffer range. If they are different by more than 10 then this is considered a bad buffer.
Concept #3: Method 2 for Creating a Buffer.
The second method in creating a buffer is mixing a strong acid with a weak base. In this case since we have a strong species mixing with a weak species then we must make sure the weak species is higher in amount.
Concept #4: Method 3 for Creating a Buffer.
The third method in creating a buffer is mixing a strong base with a weak acid. In this case since we have a strong species mixing with a weak species then we must make sure the weak species is higher in amount.
Example #1: Which of the following combinations can result in the formation of a buffer?
a) 0.01 moles HClO (hypochlorous acid) and 0.05 moles of NaOH.
b) 0.01 moles HClO (hypochlorous acid) and 0.05 moles of HCl.
c) 0.01 moles HClO (hypochlorous acid) and 0.05 moles of NH3.
d) 0.01 moles HClO (hypochlorous acid) and 0.001 moles of NaOH
Practice: Which of the following combinations can result in the formation of a buffer?
Practice: A buffer solution is comprised of 50.0 mL of a 0.100 M HC2H3O2 and 60.0 mL of a 0.100 M NaC2H3O2. Which of the following actions would completely destroy the buffer?
Concept #5: The Henderson – Hasselbalch Equation.
Whenever we have a buffer we can skip the ICE Chart and use the Henderson Hasselbalch.
Example #2: What is the pH of a solution consisting of 2.75 M sodium phenolate (C6H5ONa) and 3.0 M phenol (C6H5OH). The Ka of phenol is 1.0 x 10-10.
Practice: Calculate the pH of a solution formed by mixing 200 mL of a 0.400 M C2H5NH2 solution with 350 mL of a 0.450 M C2H5NH3+ solution. (Kb of C2H5NH2 is 5.6 x 10 -4).
Example #3: What is the buffer component concentration ratio, [Pr - ] / [HPr] , of a buffer that has a pH of 5.11. (The Ka of HPr is 1.30 x 10-5).
Example #4: Over what pH range will an oxalic acid (H2C2O4) / sodium oxalate (NaHC2O4) solution work most effectively? The acid dissociation constant of oxalic acid is 6.0 x 10-2.
a) 0.22 – 2.22 b) 1.00 – 3.00 c) 0.22 – 1.22 d) 2.0 – 4.0
Practice: Determine how many grams of sodium acetate, NaCH3CO2 (MW: 82.05 g/mol), you would mix into enough 0.065 M acetic acid CH3CO2H (MW: 60.05 g/mol) to prepare 3.2 L of a buffer with a pH of 4.58. The Ka is 1.8 x 10-5.
Example #5: Which weak acid-conjugate base combination would be ideal to form a buffer with a pH of 4.74.
a) Cyanic acid and Potassium cynate (Ka = 4.9 x 10-10)
b) Benzoic acid and Lithium benzoate (Ka = 6.3 x 10-5)
c) Acetic acid and Sodium acetate (Ka = 1.7 x 10-5)
d) Ammonium chloride and Ammonia (Ka = 5.56 x 10-10)
e) Formic acid and Cesium formate (Ka = 1.7 x 10-4)
Practice: A buffer solution is made by combining a weak acid with its conjugate salt. What will happen to the pH if the solution is diluted to one-fourth of its original concentration?